1. Field of the Invention
The present invention relates to a microwave plasma CVD apparatus which makes it possible to effectively form a functional amorphous film on a substrate, which is eligible to use as a constituent semiconductor member of semiconductor devices, electrophotographic photosensitive devices, image input line sensors, image pickup devices, photovoltaic devices, other various electronic and optical devices, etc. More particularly, the present invention relates to an improved microwave plasma CVD apparatus provided with a microwave transmissive window composed of specific sintered ceramics comprising alpha-alumina as the main constituent which enables one to continuously form an amorphous semiconductor film on a substrate over a long period of time without the microwave transmissive window being deteriorated.
2. Related Background Art
Heretofore, there have been proposed a number of amorphous semiconductor deposited films such as amorphous silicon deposited films respectively composed of an amorphous silicon material compensated with hydrogen atoms (H) or/and halogen atoms (X) such as fluorine, chlorine, etc. (hereinafter referred to as "a-Si(H,X)"). Some of such films have been put to practical use as a constituent element member of semiconductor devices, electrophotographic photosensitive devices, image input line sensors, image pickup devices, photovoltaic devices, other various electronic and optical devices.
It is known that such semiconductor deposited films may be obtained by means of a sputtering process, a thermal-induced CVD, a light-induced CVD process or a plasma CVD process, that is, a glow discharge decomposition process in other words, in which a deposited film is formed on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum by decomposing a film-forming raw material gas with a glow discharge using a direct current (DC) energy, a high frequency energy or a microwave energy. In recent years, the public attention has been focused on the glow discharge decomposition process using a direct current energy, a high frequency energy or a microwave energy at industrial level since an amorphous semiconductor deposited film may be relatively easily formed at a relatively low substrate temperature while controlling the quality of the film. Particularly, the glow discharge decomposition process using a microwave energy (that is, microwave plasma CVD process) has been evaluated as being the most appropriate since it is remarkably advantageous over other plasma CVD processes with the points that film deposition rate is high and utilization efficiency of a film-forming raw material gas is high.
There have been made a number of proposals utilizing the above advantages of the microwave plasma CVD process. For example, U.S. Pat. No. 4,504,518 (hereinafter referred to as Literature 1) describes a microwave plasma CVD process of forming an amorphous semiconductor alloy film on a substrate by coupling a microwave energy into a substantially enclosed reaction vessel containing the substrate, at the same time introducing into the reaction vessel at least one reaction gas of SiH.sub.4, SiF.sub.4, GeH.sub.4, etc. to form a glow discharge plasma within the reaction vessel to form reaction gas species from the reaction gas, and evacuating the reaction vessel to a deposition pressure so as to provide for the deposition of an amorphous semiconductor alloy film from the reaction gas species onto the substrate at high deposition rate with high reaction gas conversion efficiency. Literature 1 also describes that the resulting amorphous semiconductor alloy film may be controlled to a conduction of p-type or n-type by adding a dopant gas during the formation thereof. In addition, Japanese Laid-open patent application 60(1985)-186849 (U.S. Ser. No. 580,086) (hereinafter referred to as Literature 2) describes a microwave plasma CVD process and a microwave plasma CVD apparatus for producing an electrophotographic device. The microwave plasma CVD apparatus in Literature 2 includes a discharge space wherein a plurality of cylindrical substrates are arranged to surround a microwave introducing means, thereby highly enhancing the gas utilization efficiency.
Now, in order to effectively form a desirable amorphous semiconductor film by the microwave plasma CVD process while making full-use of the above-mentioned advantages, the microwave transmissive window as the microwave introducing means through which a microwave energy is applied into the film-forming chamber is one of the essentially important factors. Hitherto, there have been used for the microwave transmissive window, materials having a low dielectric constant (E) and a low dielectric loss angle (tan .delta.) to prevent transmission loss of the microwave as much as possible. Such materials are berylia (BeO), polytetrafluoroethylene, alumina ceramics, etc.
It is required for the microwave transmissive window to have sufficient resistances to, discharged heat radiation, to temperature rise at the window due to absorption of microwave and also to thermal impact. Other than these requirements, it is also required for the microwave transmissive window to have a sufficient vacuum retentivity. Further in addition, it is required for the microwave transmissive window to maintain its microwave transmission without being reduced even when a film is deposited on the surface thereof facing to the discharge space.
The conventional microwave transmissive windows made of those materials above mentioned are not satisfactory since they do not sufficiently meet the above requirements. In view of this, there have been made various proposals of the microwave transmissive window.
For example, U.S. Pat. No. 4,785,763 (hereinafter referred to as Literature 3) describes a microwave plasma CVD apparatus provided with a microwave introducing means comprised of laminated two or more microwave transmissive plates made of a dielectric material wherein the face of the outermost transmissive plate to become faced to the deposition space is of a toughened surface having a roughness of 1.5 .mu.m to about 1 cm for the height between the projection and the depression by the arithmetic mean for at least selected ten points. Literature 3 describes advantages provided by using said microwave introducing means that an amorphous material deposited on the roughened surface is hardly crystallized even when its temperature is elevated and a microwave energy is effectively introduced into the deposition space without reflected at the microwave transmissive plate even in the case of repeating the film forming process.
Other than this, U.S. Pat. No. 4,930,442 (hereinafter referred to as Literature 4) describes a microwave plasma CVD apparatus provided with an improved microwave transmissive window made of alumina ceramics containing glassy component such as SiO.sub.2, CaO and MgO in an amount of 1 wt. % to 10 wt. % and substantially as other component .alpha.-alumina. Literature 4 describes that said microwave transmissive window excels in heat resistance, heat impact resistance, vacuum retentivity and microwave power transmittance characteristics.
These improved techniques relative to the microwave introducing window of the microwave plasma CVD apparatus has made it possible to form a relatively thick photoconductive material at a relatively high deposition rate and with relatively high raw material gas utilization efficiency.
An example of such known microwave plasma CVD apparatus is of the constitution shown in FIG. 4 and FIG. 5. FIG. 4 is a schematic explanatory view illustrating the constitution of the known microwave plasma CVD apparatus, and FIG. 5 is a schematic cross section view taken along line X--X of the apparatus shown in FIG. 4.
In FIGS. 4 and 5, numeral reference 401 stands for a substantially enclosed reaction chamber (film-forming chamber in other words) having a vacuum tight structure, the inside of which can be evacuated to a vacuum of 1.times.10.sup.-7 Torr or less. The reaction chamber 401 is so structured that it serves as a cavity resonator capable of resonating with a frequency from a microwave power source (not shown) to initiate discharge by way of self-excited discharge without using a discharging trigger or the like. Numeral reference 402 stands for a microwave introducing window comprising the foregoing microwave introducing means of Literature 3 or the foregoing microwave transmissive window of Literature 4 which is capable of transmitting a microwave power from the microwave power source (not shown) into the reaction chamber 401. The microwave introducing window 402 is arranged to a waveguide 403 so as to vacuum-tight the inside of the reaction chamber 401 and to isolate the inside of the waveguide 403 from the inside of the reaction chamber 401. The waveguide 403 is connected to the microwave power source (not shown) through a stab tuner (not shown) and an isolator (not shown). The waveguide 403 comprises a rectangular-shaped portion extending from the microwave power source (not shown) to the vicinity of the reaction chamber 401 and a cylindrically-shaped portion situated in the reaction chamber 401.
The reaction chamber 401 is provided with an exhaust pipe 404 which is connected to an exhaust device (not shown) through an exhaust valve (not shown). Numeral reference 405 stands for cylindrical substrates on each of which a film is to be formed. Each of the cylindrical substrates 405 is supported on a substrate holder 405' arranged on a rotary shaft 410 connected to a driving motor 411 through a driving mechanism. Numeral reference 406 stands for a discharge space surrounded by the cylindrical substrates 405. Numeral reference 408 stands for a gas feed pipe for supplying a raw material gas into the discharge space 406. The gas feed pipe 408 is connected to gas reservoirs (not shown). Each of the substrate holders 405' is provided with an electric heater 407 for heating the cylindrical substrate 405 positioned thereon.
The formation of a deposited film in this microwave plasma CVD apparatus in order to prepare an electrophotographic photosensitive device is carried out in the following manner.
There are firstly provided a plurality of well-cleaned cylindrical substrates 405. Each of these cylindrical substrates 405 is positioned on the corresponding substrate holder 405' of the reaction chamber 401. After the reaction chamber 401 being closed, the reaction chamber 401 is evacuated to bring the inside to a vacuum of less than 1.times.10.sup.-7 Torr through the exhaust pipe 404 by means of the exhaust device (not shown). Subsequently, each of the cylindrical substrates 405 is heated to a desired temperature by means of the electric heater, and is maintained at this temperature. Film-forming raw material gases (for example, silane gas (SiH.sub.4) and hydrogen gas (H.sub.2) in the case of forming an amorphous silicon film) are introduced into the reaction chamber 401 through the gas feed pipe 408 at respective predetermined flow rates. Simultaneously, the microwave power source (not shown) is switched on to generate a microwave having a frequency of not less than 500 MHz, preferably 2.45 GHz, followed by transmission through the waveguide 403 and the microwave introducing window 402 into the reaction chamber 401. In this way, the film-forming raw material gases are excited and decomposed by the action of the microwave energy in the discharge space 406 surrounded by the cylindrical substrates 405, to produce neutral radical particles, ion particles, electrons, etc. which are reacted with each other to cause the formation of a deposited film on each of the cylindrical substrates 405. During this process, each of the cylindrical substrates 405 is rotated by rotating the rotary shaft 410 by means of the driving motor 411. As a result, there is formed a deposited film uniformly on the entire surface of each of the cylindrical substrates.
In the case of forming a photoconductive material by a microwave plasma CVD process in order to produce an electrophotographic photosensitive device by using the known microwave plasma CVD apparatus shown in FIGS. 4 and 5 which is provided with a microwave introducing window comprising the foregoing microwave introducing means of Literature 3 or the foregoing microwave transmissive window of Literature 4, the microwave introducing window comprising the foregoing microwave introducing means of Literature 3 or the foregoing microwave transmissive window of Literature 4 is practically effective to a certain extent.
However, as a result of experimental studies of the microwave introducing window by the present inventors, the following facts were found. That is, the microwave introducing window is not satisfactory particularly in durability upon repeated reuse. Particularly, the microwave introducing window comprising the foregoing microwave introducing means of Literature 3 or the foregoing microwave transmissive window of Literature 4 is still problematic upon repeated reuse by repeating the reuse cycle comprising use for film formation, removal of the deposited film and reuse for the next film formation. That is, as the repetition of the reuse cycle thereof increases, there are provided problems that discharge is hardly caused, discharge is not stably maintained, and the microwave introducing window becomes deteriorated particularly in terms of durability. Therefore, there is still a subject for the microwave introducing window comprising the foregoing microwave introducing means of Literature 3 or the foregoing microwave transmissive window of Literature 4 to be improved particularly not only in view of durability upon repeated reuse but also in view of the characteristics as the microwave introducing means. And, this subject is very important in order to mass-produce a desirable electrophotographic photosensitive device of high quality with high yield and at a reduced production cost.
Explaining the above situation in more detail, in the case of producing an amorphous silicon electrophotographic photosensitive device using the above microwave plasma CVD apparatus, it usually takes at least two hours for the formation of its photoconductive layer wherein a film is deposited at a relatively high thickness on the surface of the microwave introducing window which is exposed to plasma. Therefore, the surface of the microwave introducing window having such deposited film thereon is cleaned to remove the deposited film by means of an alkali etching technique or a sand blasting technique after every film formation cycle, and the microwave introducing window thus cleaned is subjected to reuse for the next film formation cycle. As above described, as the repetition of the reuse cycle of the microwave introducing window in this way increases, the microwave introducing introducing window itself not only becomes gradually deteriorated but also the surface thereof is gradually shaved off, and the microwave introducing window is contaminated with alkali in the case of performing alkali etching in order to remove the film deposited on the surface thereof, to thereby cause the following problems. That is, firstly, discharge becomes hardly caused. Particularly, in this case, it is necessary to apply excessive microwave power in order to cause and continue discharge, but the resulting film becomes accompanied by defects and as a result, the yield is reduced. Secondly, stable discharge cannot be maintained. As above described, it takes at least two hours for the formation of an photoconductive layer in order to produce an amorphous silicon electrophotographic photosensitive drum, wherein discharge is necessary to be maintained in a stable state during the formation of the photoconductive layer. However, when discharge is intermittently gone out or is caused in an unstable state, the resulting film becomes accompanied by defects and as a result, the yield is reduced. Thirdly, the durability of the microwave introducing window against the severe conditions upon film formation becomes deteriorated. In this case, the microwave introducing window sometimes becomes damaged or broken in the worst case during discharging.